Hydrogen-selective membranes

Consider the following reaction as representative of this type and the rates of the forward and reverse reactions  

The rate of transport of hydrogen across the membrane of area As in units of mol/time is 

The reaction can be considered to take place in the volume above the membrane. Only hydrogen is transported through the membrane, whereupon it leaves the lower volume via convective flow. Unconverted alkane A, the product alkene B, and hydrogen are also converted out of the volume above the membrane. Consider both the volumes above and below the membrane to be well mixed. 

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Another example of the use of highly hydrogen-selective membranes in the petrochemical industry is the separation of hydrogen from carbon monoxide/ hydrogen mixtures to obtain the correct ratio of components for subsequent synthesis operations. 

Figure 9.5 Hydrogen-selective membrane combining steps 1 + 2 + 3 in precombustion decarbonization. Starting from synthesis gas, for example, after gasification of coal, step 2 + 3 may be combined in the membrane unit. After WCS, the membrane unit may perform step 3, only.

In the case of C02-selective membranes in precombustion processes, the fuel heating value remains at the high-pressure retentate side (H2 and unconverted fuel) in the WGS separation process. The power-cycle efficiency for natural-gas-fuelled GTCC including C02-selective membranes in the WGS reactor appears less pressure dependent compared to hydrogen-selective membranes, due to the lower amount of C02 produced in the MSR + WGS reactions [24]. A relative simple simulation for precombustion capture made (based on C02/H2 selectivity of 50) [24], suggests that... 

For operation in a steam reformer, membranes must be found with a proper balance between permeance and selectivity. Ideally, a membrane with both high selectivity and high permeance is required, but one may expect on forehand that, typically, attempts to maximise one are compromised by a reduction in the other. State-of-the-art hydrogen-selective membranes were already discussed in chapter 1 and the reader is referred to that for more information on suitable membrane types. 

For thermodynamical reasons equilibrium conversion reaches 90% only at a temperature as large as 1000°C. This makes the process energetically very unfavourable. By removing the produced hydrogen selectively from the reaction zone (for example by using hydrogen selective membranes), the same conversion can be attained at 600-700°C. When this can be realised environmental benefits are evident. 

It is helpful to think of a simple membrane process as shown in Figure 8.1. A hydrogen-selective membrane is sealed within a housing (pressure vessel) to make a membrane module. The feed stream enters the membrane module, and hydrogen selectively permeates the membrane. The hydrogen-depleted raffinate stream exits the membrane module as does the permeate stream (enriched in hydrogen). The hydrogen partial pressure in each stream is denoted by PU2 where the subscripts f,... 

Figure 8.11. Schematic showing integration of a hydrogen-selective membrane with a steam...

In sub-configuration (A) hydrogen will permeate through the hydrogen selective membrane tube under the influence of a pressure difference over the membrane and it will be carried away with an inert sweep gas (steam). The partial pressure of hydrogen in the reaction mixture will decrease cmd the equilibrium will shift to the product side. 

The DOE Fossil Energy cost and performance targets for hydrogen selective membranes are summarized in Table 12.1 below. 

Fig. 16.1 Temperature range of application of various hydrogen selective membranes/materials. Schematic prepared based on reported membrane performance or material stability data...

Ku, A. Y. et al.. Membrane performance requirements for carbon dioxide capture using hydrogen-selective membranes in integrated gasification combined cycle (IGCC) power plants. Journal of Membrane Science, 2011. 367(1) 233-239. 

The more interesting configurations for a hydrogen selective membrane are then ... 

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